The secondary aluminum cast alloy industry uses mixed feedstock, including, Al--Mg scrap from 2xxx, 5xxx, 6xxx, and 7xxx alloys used in the aerospace, body panel and engine parts industry. It is typically "clean scrap" and is considered premium feedstock for remelting into secondary cast alloys.
These alloys usually contain up to about 3% magnesium and must be "demagged" (so that the magnesium level is reduced to about 0.1 to about 0.6% before use) for use in casting alloys.
Current technology offers essentially four procedures for magnesium removal:
1) oxidation of the magnesium during melting of the scrap;
2) vacuum distillation of the magnesium from the bulk scrap during melting;
3) reaction of the magnesium with chlorine; and
4) displacement of magnesium with iron.
However, economic and practical considerations eliminate procedures 2) and 4) because of requirements for additional processing steps or the need for expensive vacuum equipment.
While the most popular of the current technologies for removing magnesium is by chlorination with gaseous or solid sources of Cl, there is an initiative by the U.S. Environmental Protection Agency to develop and prescribe recovery processes that do not present the environmental and safety concerns of chlorination processes. With this initiative in mind, the ideal approach is to remove and recover the magnesium from the Al--Mg alloy without chlorination using a technology that is inherently safer and has a minimum of processing steps.